Just going by experiences with my own dogs, it could be an abcess from a grass seed, or maybe an interdigital cyst. If it were one of mine I would be showing it to my vet. If it is rather swollen then chances are it will burst and that's when you will need ointment for it, you wouldn't want it to become infected. If it is between more than two toes and she is licking it a lot, I mean to the point where the hair changes colour, it may have become a habit. I've known a Sheltie breeder friend here to resort to rinsing such feet in diluted Listerine mouth wash(don't know whether you have it in your country), the smell and taste puts them off further licking. Good luck with it.